AR-15s are available with both commercial and mil-spec buffer tubes, which have slightly different dimensions, particularly diameter. You need to match the stock to the buffer tube.

The intersting part is that two types have radically different lengths of pull. A basic mil-spec stock setup (e.g., Magpul CTR) has an inch less length of pull than a commercial one (about 13.5" vs. 14.5"). This isn't a big deal if you're using an Aimpoint or some other sight with unlimited eye relief, but it can create trouble with a magnifying scope. The mil-spec stock might be long enough to see the entire field of view when shooting prone. There are, however, mil-spec stocks that add additional length of pull for compatibility with magnifying optics, such as the Magpul ACS.

Tags: AR-15